Method and system for storing duplicate data

ABSTRACT

During recovery of a master system, a mismatch between data in the master system and that in a backup system is obviated quickly to shorten time consumed before resumption of operation of the master system. The backup system controls, as second difference information, update data generated over a substitutive operation period between occurrence of a disabled state of the master system and recovery thereof and when the master system is enabled to operate, a range of addition of first difference information inside the master system and the second difference information or only a range of the second difference information is copied to the master system to eliminate the data mismatch.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to data duplicating techniques andmore particularly, to a technique effectively applicable to a techniqueof holding data between a plurality of mutually independent informationprocessing systems in duplicate by using an asynchronous or synchronousdata copy technique.

[0003] 2. Description of the Related Art

[0004] For example, in users such as banking organs and insurancecompanies in which a large capacity of data is held and the loss of dataseriously affects the execution of business affairs and social life,taking a backup of the data has hitherto been practiced and besides, inrecent years, it has been desired by learning a good lesson from naturalcalamities and accidents that the data be saved in a backup system laidat a remote location.

[0005] More particularly, in order to realize preservation of data andguarantee of operation continuity, update data on a master system iscopied to a backup system laid at a remote location and in the eventthat the master system stops operating owing to a disaster or a fault,the operation is switched to the backup system to ensure continuity ofoperation.

[0006] As regards data preservation based on data duplication betweenremote locations, a technique disclosed in, for example, JP-A-8-509565is available. According to the disclosed method, data in a master systemis reflected, as it is, on a backup system to maintain a “mirror state”and when operation of the master system is disabled, the operation isswitched over to the backup system to thereby make resumption of thesystem operation easier. The method for backup copy to the master systemcan be classified principally into two kinds of “synchronous type” and“asynchronous type” from the standpoint of the chance to update the databetween the master system and the backup system.

[0007] In a technique of the synchronous type, when a request for dataupdate is made by a host of the master system, update data is firstwritten to a storage device of the master system and write of the updatedata to a storage device of the backup system is then carried out; andwhen receiving a write end notice from the backup system, the storagedevice of the master system submits a final write end report to the hostof the master system, so that synchronization of the data update betweenthe master system and the backup system can always be maintained.

[0008] Contrarily, in a technique of the asynchronous type, when arequest for data update is made by the host of the master system, awrite end report is submitted to the host of the master system at thetime that write of update data to the storage device of the mastersystem is completed but for this data update, execution of data updateapplied to the backup system is delayed, that is, done asynchronously.

[0009] In the case of the asynchronous copy type, a temporary differencein data takes place before the data updated in the master system isreflected on the backup system. Accordingly, a state of the dataunreflected on the backup system must be managed or controlled. Forexample, in a method disclosed in JP-A-10-198607, a difference controltable indicating the presence or absence of a difference every logicaltrack is provided and update data is copied to the backup system on thebasis of the difference.

[0010] In the event that the master system is disabled temporarily owingto, for example, a serious disaster while the “mirror state” beingmaintained normally, the “mirror state” is sustained by virtue of thesynchronousness of data update in the “synchronous type”. In this case,part of data concerning the final update is not reflected possibly onthe remote system depending on the timing but matching does not matter.In the “asynchronous type”, however, the update data is reflectedasynchronously on the backup system and as a result, a mismatch occursbetween the master system and the backup system. In order to recoverfrom the mismatched condition, all of the data in the backup system mustbe copied to the master system when the master system recovers from thestop state due to a disaster or fault and much time is required forcopying a large capacity of data, thus eventually delaying time for themaster system to recover.

[0011] A method for recovering from the mismatched state is disclosedin, for example, JP-A-6-290125. In the disclosed method, update data issubjected to wait queuing so as to be copied to the backup system insequence of update, thus maintaining matching. When recovering from afault, recovery from the mismatched state is made on the basis of apending write wait queue inside the backup system.

[0012] Further, a technique is disclosed in U.S. Pat. No. 5,857,208,according to which in a system for duplicating data between a localsystem and a remote system, a backup unit such as a magnetic tape deviceis connected to the remote system, difference data between the localsystem and the remote system, which difference data is generated whendata copy from the local system to the remote system is stopped to copydata to the backup unit at a specified time point, is managed orcontrolled by a track status provided on the local system side, andafter completion of backup by the remote system, the difference data iscopied from the local system to the remote system to permit recovery ofmatching.

SUMMARY OF THE INVENTION

[0013] In the event that the master system stops owing to a calamity ora fault, operation must be switched over to the backup system tocontinue business affairs until the master system recovers. In the caseof the asynchronous copy, a difference from the master system (called afirst difference) is generated at the time that operation is switchedover to the backup system as described above and as business affairscontinue with the backup system, data unreflected on the master systemrepresenting a second difference is then generated. The amount of thesecond difference differs depending on time for the master system torecover and the first and second differences may coincide with eachother partly or totally. In the method of copying all data of the backupsystem to the master system for the sake of performing recovery from amismatched state when the master system recovers, much time is requiredfor recovering from the mismatched state and as a result, time for themaster system to recover is delayed.

[0014] An object of the invention is to quickly eliminate a mismatchbetween data of the master system and that of the backup system when themaster system recovers so as to shorten time consumed before the mastersystem resumes operation.

[0015] Another object of the invention is to shorten time consumed forrecovering from a mismatch of data between individual informationprocessing systems that accompanies operation stop and operationresumption of part of the information processing systems, in a dataduplicating system that realizes data guarantee and operation continuityguarantee by holding the same data between the plurality of informationprocessing systems.

[0016] Still another object of the invention is to shorten time consumedbetween operation stop and operation resumption of part of informationprocessing systems, in a data duplicating system that realizes dataguarantee and operation continuity guarantee by holding the same databetween the plurality of information processing systems.

[0017] According to the present invention, in a data duplicating methodthat connects a first information processing system comprised of a firsthost computer and a first storage device and at least one secondinformation processing system comprised of a second host computer and asecond storage device through a data transfer path and holds the samedata in duplicate in the first and second information processing systemsby copying first update data generated in the first informationprocessing system to the second information processing system, thesecond information processing system possesses difference controlinformation for identifying second update data generated in the secondinformation processing system that takes over and executes a process ofthe first information processing system when the first informationprocessing system stops operating, and after resumption of operation ofthe first information processing system, the second update data isselectively copied to the first information processing system on thebasis of the difference control information.

[0018] According to the invention, in a data duplicating method thatconnects a first information processing system comprised of a first hostcomputer and a first storage device and at least one second informationprocessing system comprised of a second host computer and a secondstorage device through a data transfer path and holds the same data induplicate in the first and second information processing systems byasynchronously copying first update data generated in the firstinformation processing system to the second information processingsystem and having, in the first information processing system, firstdifference control information for identifying the first update data notcopied to the second information processing system, the secondinformation processing system possesses second difference controlinformation for identifying second update data generated in the secondinformation processing system that takes over and executes a process ofthe first information processing system when the first informationprocessing system stops operating, and after resumption of operation ofthe first information processing system, data in a range specified bythe first and second difference control information is selectivelycopied to the first information processing system.

[0019] According to the invention, in a data duplicating systemcomprising a first information processing system comprised of a firsthost computer and a first storage device, at least one secondinformation processing system comprised of a second host computer and asecond storage device and a data transfer path through which datatransfer between the first and second information processing systems iscarried out, whereby the data duplicating system holds the same data induplicate in the first and second information processing systems bycopying first update data generated in the first information processingsystem to the second information processing system through the datatransfer path, the second information processing system includesdifference control information for identifying second update datagenerated in the second information processing system while taking overand executing a process of the first information processing system whenthe first information processing system is disabled to operate, and thefunction to selectively copy the second update data of the secondinformation processing system to the first information processing systemon the basis of the difference control information when the firstinformation processing system is enabled to operate.

[0020] According to the invention, in a data duplicating systemcomprising a first information processing system comprised of a firsthost computer and a first storage device, at least one secondinformation processing system comprised of a second host computer and asecond storage device and a data transfer path through which datatransfer between the first and second information processing systems iscarried out, whereby the data duplicating system holds the same data induplicate in the first and second information processing systems byasynchronously copying first update data generated in the firstinformation processing system to the second information processingsystem through the data transfer path, the first information processingsystem includes first difference control information for identifying thefirst update data not copied to the second information processingsystem, and the second information processing system includes seconddifference control information for identifying second update datagenerated in the second information processing system while taking overand executing a process of the first information processing system whenthe first information processing system is disabled to operate and thefunction to selectively copy data in a range specified by the first andsecond difference control information to the first informationprocessing system when the first information processing system isenabled to operate.

[0021] More specifically, in an example of data duplicating systemcomprising a first system comprised of a host computer and a storagedevice, a plurality of second systems each comprised of a host computerand a storage device and a data transfer path through which datatransfer between the first and second systems is carried out, wherebyupdate data applied to the first system is copied asynchronously orsynchronously to the second system to duplicate data and the secondsystem is permitted to continue a process when the first system isdisabled to operate, the first system has the function to control, asfirst difference information, a difference generated when the updatedata applied to the first system is copied to the second system asnecessary, and the second system has the function to control, as seconddifference data, update data applied to the second system that isgenerated before recovery of the first system from a disabled statefollowing occurrence of the disabled state and to asynchronously copy arange of the first difference information and second differenceinformation or only a range of the second difference information to thefirst system when the first system is enabled to operate.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a block diagram showing an example of construction of aninformation processing system exemplifying a data duplicating systemadapted to carry out a data duplicating method according to anembodiment of the invention.

[0023]FIG. 2 is a conceptual diagram showing an example of constructionof a difference control table used in the information processing systemaccording to an embodiment of the invention.

[0024]FIG. 3 is a flowchart showing an example of an operation of theinformation processing system according to the embodiment of theinvention.

[0025]FIG. 4 is a flowchart showing an example of an operation in theinformation processing system according to the embodiment of theinvention.

[0026]FIG. 5 is a flowchart showing an example of an operation in theinformation processing system according to the embodiment of theinvention.

[0027]FIG. 6 is a flowchart showing an example of an operation in theinformation processing system according to the embodiment of theinvention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0028] Embodiments of the present invention will now be described ingreater detail with reference to the accompanying drawings.

Embodiment 1

[0029]FIG. 1 is a block diagram showing an example of construction of aninformation processing system exemplifying a data duplicating systemthat practices a data duplicating method according to an embodiment ofthe invention.

[0030] As exemplified in FIG. 1, the information processing system inthe present embodiment comprises two systems of master system 100 andbackup system 400.

[0031] The master system 100 includes a host computer 110, disk controlunit 120 and a disk storage device 230. The disk control unit 120includes channel I/F's 140 and 141, a channel processor 150, a cachememory 160, a common memory 170, an internal bus 240, a drive processor200 and a drive I/F 210 and is coupled to the host computer 110 througha channel path 130. The disk storage device 230 is coupled to the driveI/F 210 through a drive transfer path 250. The channel processor 150inside the disk control unit 120 functions during datatransmission/reception to/from the host computer 110 and at the timethat update data from the host computer 110 is transferred to the backupsystem 400 by using the channel I/F 141. The drive processor 200 insidethe disk control unit 120 functions during data transmission/receptionto/from the disk storage device 230 carried out through the drive I/F210.

[0032] A difference control table 180 inside the common memory 170 isupdated/referenced by the channel processor 150 so as to be used tocontrol data unreflected on the backup system 400. A pair conditioncontrol table 190 inside the common memory 170 is used to control copycondition.

[0033] More particularly, when receiving update data from the hostcomputer 110, the disk control unit 120 of the master system 100according to the present embodiment carries out an asynchronous copyprocess in which the disk control unit reports write completion to thehost computer 110 at the time that a process for writing the update dataof interest to the cache memory 160 inside the disk control unit 120 iscompleted and proceeds with a process for copying the update data ofinterest to the backup system 400 through the channel I/F 141 and aninterface cable 600 at a later arbitrary chance.

[0034] The backup system 400 has the construction/function quite similarto that of the master system 100 and includes a host computer 410, adisk control unit 420 and a disk storage device 530, the unit 420 anddevice 530 being operative under the command of the host computer 410.In the backup system 400, data transmission/reception to/from the mastersystem 100 is carried out through a channel I/F 440 connected to theinterface cable 600 and data transmission/reception to/from the hostcomputer 410 is carried out through a channel I/F 441.

[0035] In the case of the present embodiment, the disk control unit 420has a common memory 470 in which a difference control table 480 isprovided. The difference control table 480 is updated/referenced by thechannel processor 450 through an internal bus 540 and during stop ofoperation of master system 100 due to, for example, a fault, it controlsdifference data generated by update data from the host computer 410.

[0036] Data transfer between the master system 100 and the backup system400 is performed through the interface cable 600 and inquiries betweenthe host computers 110 and 410 are made by way of a communication cable700 between the hosts. In case the backup system 400 is arranged at aremote location, the interface cable 600 and communication cable 700between the hosts can be implemented with optical fiber, a communicationline or an information communication medium such as an electric wave ofradio communication that is adapted to connect the two systems.

[0037]FIG. 2 is a conceptual diagram showing an example of structure ofeach of the difference control tables 180 and 480 used in theinformation processing system according to the present embodiment. Inthe present embodiment, the I/O (input/output) associated with the hostcomputer 110 and disk control unit 120 and the I/O associated with thehost computer 410 and disk control unit 420 are addressed by logicaldevice number (DEV No.), logical cylinder number (CYL No.) and logicalhead number (HD No.). The logical device number (DEV No.) can takevalues in the range of from 0 to m, the logical cylinder number (CYLNo.) can take values in the range of from 0 to n and the logical headnumber or logical track number (HD No.) can take values of from 0 to 14.

[0038] As an example, each of the difference control tables 180 and 480exemplified in FIG. 2 has a bit map structure and in the logical headnumber, 0 to 14 is controlled as information of one word (16 bits). Tofacilitate control, the right end bit is unused. The one wordinformation is put together by (n+1) to indicate a difference state ofone logical device and (m+1) logical devices are put together toindicate difference states of all of the devices. Thus, according to thetable of FIG. 2, the presence or absence of difference can be controlledin a minimum unit of logical head number or logical track number (HDNo.).

[0039] FIGS. 3 to 6 are flowcharts showing examples of operation in theinformation processing system according to the present embodiment. Theprocess flow will be described on the basis of the flowcharts.

[0040]FIG. 3 shows the flow of processing carried out before the backupsystem 400 starts degenerative operation when the master system 100 isdisabled for processing. Firstly, in step 1000, the host computer 410detects that the master system 100 is disabled to operate. This can beimplemented by, for example, causing the host computer 410 toperiodically communicate with the host computer 110 through thecommunication cable 700 between the hosts so as to check the state.Next, in step 1010, the host computer 410 issues a SWAP request to thedisk control unit 420. This SWAP request is for causing the disk controlunit 420 normally operating for backup to conduct an operationequivalent to that of the disk control unit 120 inside the master system100. Receiving the SWAP request, the disk control unit 420 clears thedifference control table 480 in step 1020 and stores SWAP as paircondition in the pair condition control table 490 in step 1030. The paircondition represented by SWAP means a state in which while the backupsystem 400 starts the regenerative operation, the disk control unit 420controls all of update data from the host computer 410 as differencedata and does not perform copying to the master system 100.

[0041] The flow of process in the disk control unit 420 during the SWAPstate will be described with reference to a flowchart of FIG. 4.Firstly, the disk control unit 420 receives a command from the hostcomputer 410. When the command is not determined to be a write commandin step 2010, a corresponding command process is carried out in step2030. If the command is a write command, the pair condition controltable 490 is referenced in step 2040. When the pair condition is forSWAP or COPY to be described later, the program proceeds to step 2050.If the pair condition is not for COPY, the program proceeds to step2070. In the step 2050, an address of the corresponding differenceinformation is calculated from an address of a write target and bits ofinterest in the difference control table 480 are rendered ON (“1”) instep 2060. Subsequently, in the step 2070, the write data of interest isstored in the cache memory 460 and write completion is reported to thehost computer 410 in step 2080 to keep the process for write commandcontinuing. It is to be noted that the write data stored in the cachememory 460 is asynchronously stored in the disk storage device 530 bymeans of the drive processor 500.

[0042] Next, operation of the backup system 400 and master system 100when the master system 100 recovers from the disabled state will bedescribed with reference to flowcharts of FIGS. 5 and 6.

[0043] Firstly, in step 3000 of FIG. 5, the host computer 410 detectsthat the master system 100 has recovered. This can be implemented by,for example, causing the host computer 410 to periodically check thestate of the host computer 110 by using the communication cable 700between the hosts.

[0044] Next, in step 3010, the host computer 410 issues a Resync requestto the disk control unit 420. When receiving the Resync request, thedisk control unit 420 reads data in the difference control table 180inside the disk control unit 120 through the interface cable 600 in step3020. The read data is OR-ed with data in the difference control table480 and a result of the logical OR operation is stored in the differencecontrol table 480. This newly generated difference information isnecessary for maintaining matching between the master system 100 and thebackup system 400.

[0045] Subsequently, in step 3030, the pair condition in the paircondition control table 490 is changed to COPY. Next, in step 3040, adifference copy process is started. The difference copy process is a JOBexecuted asynchronously with update data from the host command. In thestep 3040, either a difference copy JOB may be started or the differencecopy JOB per se may reference the pair condition control table 490periodically so that copy may be started at a chance that the paircondition changes to COPY.

[0046] In the difference copy process, the difference control table 480is first referenced in step 3050 to search locations where bits arerendered ON. When the locations for bit ON are subsequently found instep 3060, the program proceeds to step 3070 in which the differencedata of interest is transferred to the disk control unit 120 and thetransferred bits of interest are rendered OFF (“0”).

[0047] If the locations for bit ON are not found in the step 3060, theprogram proceeds to step 3050 so that difference search may be executedagain.

[0048] The flowchart of FIG. 6 shows an example of sequence for againswitching over the process in the backup system 400 to the master system100. Firstly, in step 4000, the host computer 410 stops the I/O. This isnecessary because if a new difference is generated during the switchoversequence, matching cannot be guaranteed in respect of this portion.Next, in step 4010, a DELPAIR request issued from the host computer 410to the disk control unit 420.

[0049] When receiving the RELPAIR request, the disk control unit 420searches a difference by using the difference control table 480 in step4020 and if difference data is detected in step 4030, transfers thedifference data to the disk control unit 120 in step 4040. When thedifference data disappears totally, the program proceeds to step 4050 inwhich the pair condition in the pair condition control table 490 is setto “normal”. Next, in step 4060, completion of the DELPAIR is reportedto the host computer 410.

[0050] In response to the completion report of the DELPAIR request,control is switched over from the host computer 410 to the host computer110 in step 4070. Next, in step 4080, the host computer 110 issues apair recovery request to the disk control unit 120. Receiving the pairrecovery request, the disk control unit 120 totally clears thedifference control table 180 inside the disk control unit 120 in step4090 and changes the internal state to COPY. Through this, as in thecase of the steps 3050 to 3060 in FIG. 5, the difference copy process isstarted. Since matching of stored data is maintained between the mastersystem 100 and the backup system 400 at that time, copy of all datausually necessary for setting up the pair is unneeded. Through the aboveprocess, the master system can recover its state before stop ofoperation.

[0051] According to the data duplicating method and system according tothe present embodiment, when the master system 100 recovers from thedisabled state due to, for example, a fault so as to be enabled foroperation, matching of data between the master system 100 and the backupsystem 400 can be set up quickly by necessarily minimum data copy fromthe backup system 400 to the master system 100, thereby ensuring thattime consumed before resumption of operation of the master system 100can be shortened.

Embodiment 2

[0052] As a second embodiment of the invention, an instance will bedescribed in which when difference data representing a target of copyfrom the backup system 400 to the master system 100 is generated duringrecovery of the master system 100, difference information of thedifference control table 180 in the master system 100 is not merged.

[0053] Namely, in the second embodiment, the process in step 3020 inFIG. 5 is unneeded. Other steps are the same as those in the firstembodiment.

[0054] The present embodiment is adapted for the case where update datagenerated before stop of operation of the master system 100 isconsidered to be permitted for mismatch (presence of unreflected data)between the master system 100 and the backup system 400 depending on thetype of service affairs.

[0055] In this case, meritorious effects similar to those in the firstembodiment can be obtained and besides, because of unexecuted recoveryof difference data based on the difference control table 180 on the sideof the master system 100, time required for the copy process ofdifference data from the backup system 400 to the master system 100 canbe shorter than that in the first embodiment.

Embodiment 3

[0056] Reverting to FIG. 1, an information processing system accordingto a third embodiment of the invention will be described in whichsynchronous copy is carried out from the master system 100 to the backupsystem 400.

[0057] More particularly, in the case of the synchronous copy accordingto the third embodiment, when receiving update data from the hostcomputer 110, the disk control unit 120 of the master system 100conducts a process for writing the update data of interest to the cachememory 160 (disk storage device 230) inside the disk control unit 120and executes a process for copying the update data of interest to thebackup system 400 through the channel I/F 141 and interface cable 600.Then, after completion of write of that update data has been confirmedin both the master system 100 and the backup system 400, the diskcontrol unit 120 reports completion of write to the host computer 110representing the request originator.

[0058] Accordingly, in the third embodiment, the difference controltable 180 on the side of the master system 100 in FIG. 1 is unneeded.Further, the process in step 3020 of FIG. 5 is unnecessary. Other stepsare similar to those in the first embodiment.

[0059] In the third embodiment, too, meritorious effects similar tothose in the first embodiment can be obtained and besides, because ofnonexistence of the difference control table 180 on the side of themaster system 100 and nonexistence of recovery of difference data by thedifference control table 180, time required for the process of copyingthe difference data from the backup system 400 to the master system 100can be shorter than that in the first embodiment.

[0060] The invention made by the present inventors has been set forthspecifically on the basis of the embodiments of the invention but thepresent invention is in no way limited to the foregoing embodiments andcan be altered in various ways without departing from the gist of theinvention.

[0061] For example, in contrast to one to one correspondence between themaster system and the backup system exemplified in the foregoingembodiments, a plurality of backup systems may be provided.

[0062] Advantageously, according to the data duplicating technique ofthe present invention, when the master system recovers, a mismatchbetween data in the master system and that in the backup system can beeliminated quickly and time consumed before resumption of operation ofthe master system can be shortened.

[0063] Advantageously, according to the data duplicating technique ofthe present invention, in the data duplicating system that realizesguarantee of data and operation continuity by holding the same databetween a plurality of information processing systems in duplicate, timerequired for recovery from a mismatch of data between the individualinformation processing systems accompanying the operation stop andoperation resumption of part of the information processing systems canbe shortened.

[0064] Advantageously, according to the data duplicating technique ofthe present invention, in the data duplicating system that realizesguarantee of data and operation continuity by holding the same databetween a plurality of information processing systems in duplicate, timeconsumed between operation stop and operation resumption of part of theinformation processing systems can be shortened.

[0065] The present invention can also be applied to a technique ofholding data between a plurality of mutually independent informationsystems in multiple levels more than two.

What is claimed is:
 1. A data duplicating method that connects a firstinformation processing system comprised of a first host computer and afirst storage device and at least one second information processingsystem comprised of a second host computer and a second storage devicethrough a data transfer path and holds the same data in duplicate insaid first and second information processing systems by copying firstupdate data generated in said first information processing system tosaid second information processing system, wherein said secondinformation processing system possesses difference control informationfor identifying second update data generated in said second informationprocessing system that takes over and executes a process of said firstinformation processing system when said first information processingsystem stops operating, and after resumption of operation said firstinformation processing system, said second update data is selectivelycopied to said first information processing system on the basis of saiddifference control information.
 2. A data duplicating method accordingto claim 1 , wherein said difference control information is a bit mapthat indicates the presence or absence of completion of data duplicationof said first and second update data at a plurality of individual unitsof data storage in each of said first and second storage devices.
 3. Adata duplicating method that connects a first information processingsystem comprised of a first host computer and a first storage device andat least one second information processing system comprised of a secondhost computer and a second storage device through a data transfer pathand holds the same data in duplicate in said first and secondinformation processing systems by asynchronously copying first updatedata generated in said first information processing system to saidsecond information processing system and having, in said firstinformation processing system, first difference control information foridentifying said first update data not copied to said second informationprocessing system, wherein said second information processing systempossesses second difference control information for identifying secondupdate data generated in said second information processing system thattakes over and executes a process of said first information processingsystem when said first information processing system stops operating,and after resumption of operation of said first information processingsystem, data in a range specified by said first and second differencecontrol information is selectively copied to said first informationprocessing system.
 4. A data duplicating method according to claim 3 ,wherein said first and second difference control information are a bitmap that indicates the presence or absence of completion of dataduplication of said first and second update data at a plurality ofindividual units of data storage in each of said first and secondstorage devices.
 5. A data duplicating method that connects a firstinformation processing system comprised of a first host computer and afirst storage device and at least one second information processingsystem comprised of a second host computer and a second storage devicethrough a data transfer path and holds the same data in duplicate insaid first and second information processing systems by asynchronouslycopying first update data generated in said first information processingsystem to said second information processing system and having, in saidfirst information processing system, first difference controlinformation for identifying said first update data not copied to saidsecond information processing system, wherein said second informationprocessing system possesses second difference control information foridentifying second update data generated in said second informationprocessing system that takes over and executes a process of said firstinformation processing system when said first information processingsystem stops operating, and after resumption of operation of said firstinformation processing system, said second update data is selectivelycopied to said first information processing system on the basis of saidsecond difference control information.
 6. A data duplicating methodaccording to claim 5 , wherein said first and second difference controlinformation are a bit map that indicates the presence or absence ofcompletion of data duplication of said first and second update data at aplurality of individual units of data storage in each of said first andsecond storage device.
 7. A data duplicating method that connects afirst information processing system comprised of a first host computerand a first storage device and at least one second informationprocessing system comprised of a second host computer and a secondstorage device through a data transfer path and constantly holds thesame data in duplicate in said first and second information processingsystems by synchronously copying first update data generated in saidfirst information processing system to said second informationprocessing system, wherein said second information processing systempossesses second difference control information for identifying secondupdate data generated in said second information processing system thattakes over and executes a process of said first information processingsystem when said first information processing system stops operating,and after resumption of operation of said first information processingsystem, said second update data is selectively copied to said firstinformation processing system on the basis of said second differencecontrol information.
 8. A data duplicating method according to claim 7 ,wherein said second difference control information is a bit map thatindicates the presence or absence of completion of data duplication at aplurality of individual units of data storage in each of said first andsecond storage devices.
 9. A data duplicating system comprising a firstinformation processing system comprised of a first host computer and afirst storage device, at least one second information processing systemcomprised of a second host computer and a second storage device and adata transfer path through which data transfer between said first andsecond information processing systems is carried out, whereby said dataduplicating system holds the same data in duplicate in said first andsecond information processing systems by copying first update datagenerated in said first information processing system to said secondinformation processing system through said data transfer path, whereinsaid second information processing system includes difference controlinformation for identifying second update data generated in said secondinformation processing system while taking over and executing a processof said first information processing system when said first informationprocessing system is disabled to operate, and the function toselectively copy said second update data of said second informationprocessing system to said first information processing system on thebasis of said difference control information when said first informationprocessing system is enabled to operate.
 10. A data duplicating systemaccording to claim 9 , wherein said difference control information is abit map that indicates the presence or absence of completion ofduplication of said first and second update date at a plurality of unitsof data storage in each of said first and second storage devices.
 11. Adata duplicating system comprising a first information processing systemcomprised of a first host computer and a first storage device, at leastone second information processing system comprised of a second hostcomputer and a second storage device and a data transfer path throughwhich data transfer between said first and second information processingsystems is carried out, whereby said data duplicating system holds thesame data in duplicate in said first and second information processingsystems by asynchronously copying first update data generated in saidfirst information processing system to said second informationprocessing system through said data transfer path, wherein said firstinformation processing system includes first difference controlinformation for identifying said first update data not copied to saidsecond information processing system; and said second informationprocessing system includes second difference control information foridentifying second update data generated in said second informationprocessing system while taking over and executing a process of saidfirst information processing system when said first informationprocessing system is disabled to operate, and the function toselectively copy data in a range specified by said first and seconddifference control information to said first information processingsystem when said first information processing system is enabled tooperate.
 12. A data duplicating system according to claim 11 , whereinsaid first and second difference control information are a bit map thatindicates the presence or absence of completion of data duplication ofsaid first and second update data at a plurality of units of datastorage in each of said first and second storage devices.
 13. A dataduplicating system comprising a first information processing systemcomprised of a first host computer and a first storage device, at leastone second information processing system comprised of a second hostcomputer and a second storage device and a data transfer path throughwhich data transfer between said first and second information processingsystems is carried out, whereby said data duplicating system holds thesame data in duplicate in said first and second information processingsystems by asynchronously copying first update data generated in saidfirst information processing system to said second informationprocessing system through said data transfer path, wherein said firstinformation processing system includes first difference controlinformation for identifying said first update data not copied to saidsecond information processing system; and said second informationprocessing system includes second difference control information foridentifying second update data generated in said second informationprocessing system while taking over and executing a process of saidfirst information processing system when said first informationprocessing system is disabled to operate, and the function toselectively copy said second update data of said second informationprocessing system to said first information processing system on thebasis of said second difference control information when said firstinformation processing system is enabled to operate.
 14. A dataduplicating system according to claim 13 , wherein said first and seconddifference control information are a bit map that indicates the presenceor absence of completion of data duplication of said first and secondupdate data at a plurality of units of data storage in each of saidfirst and second storage devices.
 15. A data duplicating systemcomprising a first information processing system comprised of a firsthost computer and a first storage device, at least one secondinformation processing system comprised of a second host computer and asecond storage device and a data transfer path through which datatransfer between said first and second information processing systems iscarried out, whereby said data duplicating system holds the same data induplicate in said first and second information processing systems bysynchronously copying first update data generated in said firstinformation processing system to said second information processingsystem through said data transfer path, wherein said second informationprocessing system includes second difference control information foridentifying second update data generated in said second informationprocessing system while taking over and executing a process of saidfirst information processing system when said first informationprocessing system is disabled to operate, and the function toselectively copy data in a range specified by said second differencecontrol information to said first information processing system whensaid first information system is enabled to operate.
 16. A dataduplicating system according to claim 15 , wherein said seconddifference control information is a bit map that indicates the presenceor absence of completion of data duplication of said firs and secondupdate data at a plurality of units of data storage in each of saidfirst and second storage devices.